The present invention relates to a ceramic coating method for coating a ceramic material to a metallic substrate, to obtain a coating film excellent in durability free of film cracks and peeling, in which ceramic coating onto a metallic substrate is performed by forming a coating film by continuously changing the composition ratio of ceramic and metal and then performing heat treatment to thereby induce residual stress due to compression on the ceramic coating film surface.
Recently, a technique of ceramic coating on a metallic substrate has prevailed to improve such functions or characteristic features as heat resistance, corrosion resistance and wear resistance, and this technique has been widely applied. However, a ceramic film formed by using various conventional coating processes is fragile and likely to cause cracks and peeling. Thus, ceramic coating effect is not utilized sufficiently and such defects often lead to a trouble during the coating process. Particularly, when a coated material is exposed to a high temperature environment during its use, a thermal stress occurs as a result of a difference in thermal expansions between the metallic substrate and the ceramic coating film as well as of external stress, thereby worsening the coating condition.
In view of above points, in a composite material composed of a plurality of materials like a coating material, some trials have been made to reduce the thermal stress caused by a difference in the coefficients of linear expansion of composing material. For example, Japanese Patent Laid-open (KOKAI) Publication No. 4-214826 and No. 4-337011 disclose techniques intending to reduce the thermal stress by eliminating sudden changes of such physical properties as the coefficient of linear expansion and Young's modulus by changing the composition on the interface between two materials. Namely, both are related to the production of material having a composition being changed continuously.
The Japanese Patent Laid-open (KOKAI) Publication No. 4-214826 discloses a technique for achieving the gradient composition of two materials by an infiltration of low-melting point material into pores after producing a high-melting point material having continuously changing porosity. The Japanese Patent Laid-open (KOKAI) Publication No. 4-337011 discloses a method intended to finish into an optional shape by a plastic forming working such as extrusion, drawing and rolling working after a gradient composition block has been produced using a sintering method which relatively facilitates gradient composition and is a production method suitable for a large member having a composition changing continuously in its longitudinal dimension.
On the other hand, in some trials, the same concept is adapted to the ceramic coating material to relax the thermal stress by changing the composition on the interface between the substrate and the ceramic coating film. As a coating process for continuously changing the composition, a plasma spray coating method, a PVD method and a CVD method are known to be effective as methods which enable the coating by controlling the production conditions (e.g., Bulletin of the 4th Gradient Function Material Symposium, pp. 149, pp. 119). Additionally, it is also known that optimization of composition change is effective to relax thermal stress (the 4th Gadient Function Material Symposium, pp. 19).
As described above, various trials have already been done to prevent cracks and peeling of the film from occurring due to the thermal stress during the use of the ceramic coating film. However, in the prior art, these trials have been made from the viewpoint of reducing thermal stress caused in the coating process. On the other hand, it is generally known that the residual stress occurs depending on the heat history during the ceramic coating process. Naturally, the residual stress distribution caused in a film during the coating process is considered to affect the characteristics of film cracks and peeling. Thus, it is important to establish a coating method and a coating condition considering the residual stress in the coating process in order to form a coating film having an excellent durability free of cracks and peeling.